A trapdoor in the water tower opened when the guests approached. Thumping live music, candlelight, chatter and the sound of clinking glasses emerged, as well as a helping hand.

Inside was a round wooden space no bigger than a freight elevator, filled with about a dozen people sipping whiskey cocktails. Couples sat at five petite tables built into the cedar paneling. A young woman mixed drinks behind a bar. Above people’s heads, a two-man band — accordion and upright bass — serenaded from a platform.

But amid the revelry, the staff communicated using headsets, checking that the operation remained unnoticed outside. In the event that the police did arrive, several exit routes were planned. This was life inside the Night Heron, a decidedly illegal nightclub run by a group of adventure-minded artists in a water tower atop a vacant building in Chelsea for eight weekends in March, April and May.

The Night Heron was as exclusive as it was lawless. The only way to get in was to be handed a pocket watch by a prior guest (who had been instructed to offer minimal explanation), report to a street corner at a certain time, and call a number pasted inside the watch. Mysterious helpers led guests through one decrepit building into another and up 12 flights of stairs to the roof. The watches were taken at the door, but guests were given the chance to buy watches at the end of the night if they wanted to continue the chain of invitation.

The Heron’s architect was N. D. Austin, a 31-year-old artist known for what he calls “trespass theater.” “It’s about making the invisible visible,” he said of his philosophy.

Mr. Austin located a suitable water tower by scouring Buildings Department records for violations with egregious scaffold fines. That can indicate a neglectful landlord, he said, which meant it might be a vacant building ripe for adopting as one’s own.

One Saturday night last month, 12 guests squeezed through the trap door into the space. “The great thing about the upright bass is how it got up here,” said Dirby Luongo, one of Mr. Austin’s collaborators who played the doorman. “It’s like a ship in a bottle.”

At one table, a first date was in progress. Chelsea Cammarota, 35, explained that she and her date did not know each other well. “He sent me a photo of a clock,” she recounted. “I said, ‘I’ve seen a lot of “Law and Order.” ’ ” Nevertheless, her date, Steve Showalter, told her, “We’re going to do something fun.” But he was clueless, too, running on blind faith in the friend who had given him the watch.

Caroline and Michael Ventura, a married couple, arrived for the next seating. There were three per night, each lasting an hour and a half. Mr. Ventura said a friend had arrived at his office unannounced to give him the watch. “He placed it onto my desk, looked at me and said, ‘I will answer no questions.’ ” On the way over, Mr. Ventura and his wife wondered where the night might lead: Some place underground? Some sort of sex club?

The night’s final seating ended near 3 a.m., culminating with a loud drinking ballad from the band, the sardine-packed audience stomping and yelling along. Tipsy guests exited back through the hole like paratroopers to greet a quiet, blinking skyline.

A version of this article appears in print on , on Page A25 of the New York edition with the headline: Water Tower In Chelsea Manifests A Secret Life. Order Reprints | Today’s Paper | Subscribe